The present invention relates to a hydrocarbon conversion process. More particularly, this invention relates to the catalytic hydrocracking of hydrocarbons.
The hydrocracking of hydrocarbons is old and well-known in the prior art. These hydrocracking processes can be used to hydrocrack various hydrocarbon fractions such as reduced crudes, gas oils, heavy gas oils, topped crudes, shale oil, coal extract and tar extract wherein these fractions may or may not contain nitrogen compounds. Modern hydrocracking processes were developed primarily to process feeds having a high content of polycyclic aromatic compounds, which are relatively unreactive in catalytic cracking. The hydrocracking process is used to produce desirable products such as turbine fuel, diesel fuel, and light distillate products such as naphtha and gasoline.
The hydrocracking process is generally carried out in any suitable reaction vessel under elevated temperatures and pressures in the presence of hydrogen and a hydrocracking catalyst so as to yield a product containing the desired distribution of hydrocarbon products.
Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support. More specifically, hydrocracking catalysts comprise a hydrogenation component selected from the group consisting of Group VIA metals and Group VIII metals of the Periodic Table of Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts contain an acidic support comprising a crystalline aluminosilicate material such as Y-type aluminosilicate materials. This crystalline aluminosilicate material is generally suspended in a refractory inorganic oxide such as silica, alumina, or silica-alumina.
While the prior art has generally taught the use of molecular sieves, especially crystalline aluminosilicate materials such as type X and type Y in hydrocracking catalysts, the use of other types of molecular sieves has also been suggested in various other types of hydrocarbon conversion processes. For instance, U.S. Pat. No. 4,304,686 (Telford) discloses the use of gallium exchanged zeolites in hydrocarbon conversion reactions such as dehydrocyclodimerization reactions. U.S. Pat. No. 3,944,482 (Mitchell et al.) discloses the use of a variety of molecular sieves, of which a gallosilicate can be one, in connection with a fluidized catalytic cracking process, U.S. Pat. No. 4,620,921 (Chang et al.) discloses a zeolite material having enhanced acid, e.g. cracking, activity wherein a high silica content zeolite is hydrothermally treated in the presence of a compound of aluminum or gallium.
Regarding the hydrocracking catalyst hydrogenation components, the art has generally recognized that the preferred Group VIA metals are tungsten and molybdenum while the preferred Group VIII metals are nickel and cobalt.
The prior art has further taught that combinations of metals for the hydrogenation component, expressed as oxides and in the order of preference, are: NiO-WO.sub.3, NiO-Mo.sub.3, CoO-Mo.sub.3, and CoO-W.sub.3.
Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, indium, platinum, chromium, molybdenum, vanadium, niobium, and tantalum.
References that disclose hydrocracking catalysts utilizing nickel and tungsten as hydrogenation components, teach enhanced hydrocracking activity when the matrix or catalyst support contains silica-alumina. For instance, U.S. Pat. Nos. 4,576,711, 4,563,434, and 4,517,073 all to Ward et al., show at Table V thereof that the lowest hydrocracking activity is achieved when alumina is used in the support instead of a dispersion of silica-alumina in alumina. The lowest hydrocracking activity is indicated by the highest reactor temperature required to achieve 60 vol.% conversion of the hydrocarbon components boiling above a predetermined boiling range temperature end point to below that boiling range temperature end point.
Similarly, U.S. Pat. No. 3,536,605 to Kittrell et al. teaches the use of silica-alumina in the catalyst support when a nickel- and tungsten-containing hydrogenation component is employed.
U.S. Pat. No. 3,598,719 to White teaches a hydrocracking catalyst that can contain no silica; however, the patent does not present an example showing the preparation of a catalyst devoid of silica nor does the patent teach the preferential use of nickel and tungsten as hydrogenation metals.
As can be appreciated from the above, there is a myriad of catalysts known for hydrocracking whose catalytic properties vary widely. A catalyst suitable for maximizing naphtha yield may not be suitable for maximizing the yield of turbine fuel. Further, the degree of cracking and yield structure is also dependent upon the feedstock composition.
Catalysts having high hydrogenation activity relative to acidity yield more highly saturated products as required in distillate fuels such as jet or aviation fuel.
Reconciling hydrodenitrogenation activity with hydrocracking activity in a single hydrocracking catalyst presents a difficulty. For instance, when a feedstock having a high nitrogen content is exposed to a hydrocracking catalyst containing a high amount of cracking component, the nitrogen serves to poison or deactivate the cracking component. Another difficulty is presented when the hydrocracking process is used to maximize naphtha yields from a feedstock containing light catalytic cycle oil which has a very high aromatics content. The saturation properties of the catalyst must be carefully gauged to saturate only one aromatic ring of a binuclear aromatic compound such as naphthalene in order to preserve desirable high octane value aromatic-containing hydrocarbons for the naphtha fraction. If the saturation activity is too high, all of the aromatic rings will be saturated and subsequently cracked to lower octane value paraffins.
On the other hand, distillate fuels such as diesel fuel or aviation fuel have specifications that stipulate a relatively low aromatics content. This is due to the undesirable smoke production caused by the combustion of aromatics in diesel engines and jet engines.
Prior art processes designed to convert high nitrogen content feedstocks and produce jet fuel are usually two stage processes wherein the first stage is designed to convert organic nitrogen compounds to ammonia prior to contacting with a hydrocracking catalyst which contained a high amount of cracking component; e.g., a molecular sieve material.
For instance, U.S. Pat. No. 3,923,638 to Bertolacini et al. discloses a two-catalyst process suitable for converting a hydrocarbon containing substantial amounts of nitrogen to saturated products adequate for use as jet fuel. Specifically, the subject patent discloses a process wherein the hydrodenitrogenation catalyst comprises as a hydrogenation component a Group VIA metal and Group VIII metal and/or their compounds and a cocatalytic acidic support comprising a large-pore crystalline aluminosilicate material and refractory inorganic oxide. The hydrocracking catalyst comprises as a hydrogenation component a Group VIA metal and a Group VIII metal and/or their compounds, and an acidic support of large-pore crystalline aluminosilicate material. For both hydrodenitrogenation catalyst and the hydrocracking catalyst, the preferred hydrogenation component comprises nickel and tungsten and/or their compounds and the preferred large-pore crystalline aluminosilicate material is ultrastable Y large-pore crystalline aluminosilicate material.
Thus there is a need for a single catalyst that possesses high hydrodenitrogenation, hydrocracking, and polyaromatic saturation activity. Specifically, there is a need for a catalyst that is capable of maximizing naphtha selectivity, especially heavy naphtha having a boiling range from 180.degree. F. to 380.degree. F. and octane value.
In accordance with the present invention the hydrodenitrogenation, hydrocracking, and polyaromatic saturation activities are maximized in one catalyst when a feedstock containing highly aromatic light catalytic cycle oil is converted. Further, the process of the invention provides for increased selectivity towards high octane heavy naphtha with decreased undesirable selectivity towards C.sub.1 to C.sub.5 light gas.